A recently developed technology called epitope scanning permits the rapid and accurate delineation of continuous stretches of amino acids in a protein which constitute the sequential epitopes recognised by an antiserum raised to that protein. In the present report, we describe the use of this technique to identify the epitopes in the recombinant bovine growth-hormone (rbGH) molecule recognised by three polyclonal guinea-pig antisera and two polyclonal rabbit antisera. The results obtained show that, for guinea-pig antisera, 3 or 4, very-well-defined major continuous epitopes are present. As would be expected given the intrinsic genetic factors (major histocompatibility restriction, antigen processing and presentation) controlling the immune response in individual animals, subtle differences are evident in the precise location and relative reactivities of these epitopes in different guinea-pig antisera. Nevertheless, there is a large degree of overlap in these epitopes, such that immunodominant regions of the antigen can be clearly delineated. In a structural sense, these epitopes share a common motif in that they are sited in areas of the protein antigen with little secondary structure (loop/coil), although there is some contribution by neighbouring a-helices. For the two rabbit antisera, the response tends to be rather more heterogeneous, with recognition of more peptides and less clearly defined epitopes than was the case with the guinea-pig antiserum.Comparison of the four guinea-pig epitopes, identified by our experimental methods with computer predictions for this molecule (Jameson-Wolf antigenic index), indicate that two are strongly predicted, one is weakly predicted and one is not predicted. These observations, together with the displayed intraspecies and interspecies variation clearly indicate the limitations of these predictive methods.In conclusion, we have demonstrated that, despite the expected variation in the exact location of continuous epitopes defined by different anti-rbGH sera, there are large regions of overlap defining immunogenic core regions within the molecule. We believe that studies of this nature, together with further understanding of antigen processing and peptide presentation to immune cells, may have a role to play in the development of candidate peptide vaccines.Classically, the binding sites on protein antigens (epitopes) recognised by antibodies have been categorised as either continuous or discontinuous. Continuous epitopes are defined by amino acid residues which are sequential in the sequence of the polypeptide antigen, whereas discontinuous epitopes are formed by residues proximate to each other in the threedimensional structure of the protein, although not necessarily so in the sequence [l].Recently, however, this definition has been challenged on both theoretical [2] and experimental [3] grounds. For example, studies which have examined the cocrystallisation of protein antigen with antibody have found all epitopes on the antigen to be discontinuous (or conformational) in ...